Most of my quests will become available when you/your partner have announced their pregnancy. Any requirements beyond that will be listed below.

Aela: Complete the quest "The Silver Hand" and Aela is at least 2 months pregnant.

Mjoll: Aerin must be alive for the first and second part of the quests. Part two will start around a week after Mjoll gives birth.

Ysolda: Ysolda needs to be 3 months pregnant or more to start the quest.

Serana: Make sure Valerica has returned from the Soul Cairn and that Serana is past 1 month.